The present invention relates to a method and apparatus for receiving a radio frequency signal, and a method and apparatus for transmitting a modulation signal.
Current mobile telephone systems are mainly provided by ground based (terrestrial) cellular systems such as GSM (Global System for Mobiles), PDC 800 or AMPS (Advanced Mobile Phone System). Other mobile telephone systems include the Inmarsat-M satellite system, in which subscribers use briefcasesized mobile telephones to make calls via geo-stationary orbiting satellites.
Several new proposals for mobile satellite telephone systems are currently under development by major telecommunications companies. These global systems have been well publicised and are known commercially by the trade names ICO, IRIDIUM, GLOBALSTAR and ODYSSEY. In parallel with the launch of these new mobile satellite systems, several terminal equipment manufacturers are developing handheld mobile units for use by subscribers of these systems. Some terminal equipment manufacturers are proposing developing dual-mode handsets which operate on both the ground based cellular systems and the new mobile satellite"" systems.
WO 96/08883 discloses a dual mode telephone device which is operable on both satellite and land based cellular communication systems. The device includes one set of antenna and radio frequency circuits for receiving and transmitting signals in a satellite network, and another set of antenna and radio frequency circuits for receiving and transmitting in a terrestrial network. A dual mode frequency synthesiser provides for wide channel spacings when supplying the terrestrial radio frequency circuits and narrow channel spacings when supplying the satellite radio frequency circuits.
According to a first aspect of the present invention there is provided a method of receiving a radio frequency signal in a multiple channel radio system, in which each channel has an associated channel frequency and the channel frequencies of adjacent channels are separated by a channel spacing, the method comprising the steps of: processing the radio frequency signal in combination with a first analogue demodulating signal to produce a first analogue intermediate signal, and processing the first analogue intermediate signal in combination with a second analogue demodulating signal to produce a second analogue intermediate signal, wherein the frequency resolution of the first analogue demodulating signal is wider than the channel spacing, and the frequency resolution of the second analogue demodulating signal is finer than the frequency resolution of the first analogue demodulating signal, and the frequencies of the first and second analogue demodulating signals are adjusted in accordance with their respective frequency resolutions in order to tune the receiver to the radio frequency signal.
Adjustment of the first and second demodulating signals may tune the receiver to the channel frequency of the radio frequency signal. Alternatively, adjustment of the first and second demodulating signals may tune the receiver to a channel frequency in the vicinity of the radio frequency signal in order for a digital tuning process to further tune the receiver to the channel frequency of the radio frequency signal.
According to a second aspect of the present invention there is provided a method for transmitting a modulation signal in a multiple channel radio system, in which each channel has an associated channel frequency and the channel frequencies of adjacent channels are separated by a channel spacing, the method comprising the steps of: processing the modulation signal in combination with a first analogue modulating signal to produce an analogue intermediate signal, and processing the analogue intermediate signal in combination with a second analogue modulating signal to produce a radio frequency output signal at a channel frequency of the multiple channel radio system, wherein the frequency resolution of the second analogue modulating signal is wider than the channel spacing, and the frequency resolution of the first analogue modulating signal is finer than the frequency resolution of the second analogue modulating signal, and the frequencies of the first and second analogue modulating signals are adjusted in accordance with their respective frequency resolutions so as to change the channel frequency of the output signal.
The first or second analogue modulating or demodulating signal may vary across a range of frequencies in such a way that the possible frequency values which may be adopted by the signal have a minimum frequency separation. This minimum frequency separation is known as the frequency resolution of the signal and relates to the ability of the signal to resolve or differentiate between channel frequencies in the multiple channel radio system. The frequency resolution of the modulating or demodulating signal may be considered to be the minimum frequency jump that the modulating or demodulating signal may confidently perform. Accordingly, if the frequency resolution of a signal is widened then its ability to resolve is decreased i.e. the minimum jump that it may perform is larger. Conversely, if the frequency resolution of a signal is made finer then its ability to resolve is increased i.e. the minimum jump that it may perform is smaller.
A method in accordance with the first and/or second aspect of the invention provides an advantage that the aggregate settling time when changing channels in a transmitter or receiver may be decreased. By decreasing the settling time, a receiver or transmitter operating in accordance with the invention may be able to function within the specifications of newly proposed satellite telecommunications systems. Also, by decreasing the settling time, radio frequency circuits of the transmitter or receiver may be switched on slightly later. Consequently, the operating period of the radio frequency circuits may be reduced which in turn may reduce the power consumption in the transmitter or receiver.
In a preferred embodiment in accordance with the first aspect of the present invention, the first analogue intermediate signal is processed in combination with a further analogue demodulating signal before being processed in combination with the second analogue demodulating signal.
In a preferred embodiment in accordance with the second aspect of the present invention, the radio frequency output signal is processed in combination with a further modulating signal before being transmitted.
Preferably, the processing steps comprise mixing one signal in combination with another signal. The action of mixing one signal in combination with another signal is also referred to as frequency translation, frequency changing, or heterodyning.
Suitably, the modulating or demodulating signals are produced by frequency synthesisers. The frequency synthesisers may output a local oscillator signal which may be supplied to a mixing unit to enable the output to operate as a modulating or demodulating signal.
The first and second modulating or demodulating signals may be produced by separate frequency synthesisers.
One of the modulating or demodulating signals may be produced by a combined output of two frequency synthesisers.
In one preferred embodiment the frequency resolution of the second analogue demodulating signal or the first analogue modulating signal is equal to the channel spacing. Consequently, the second analogue demodulating signal or the first analogue modulating signal may select the individual channels in the multiple channel radio system.
In another preferred embodiment the frequency resolution of the second analogue demodulating signal or the first analogue modulating signal is greater than a channel spacing of the multiple channel radio system. In this case, selection of individual channels may be performed digitally in a base band environment.
According to a third aspect of the present invention there is provided radio frequency receiving apparatus for receiving a radio frequency signal of a multiple channel radio system, in which each channel has an associated channel frequency and the channel frequencies of adjacent channels are separated by a channel spacing, the apparatus comprising: signal generating means for producing a first analogue demodulating signal with a frequency resolution wider than the channel spacing, and a second analogue demodulating signal with a frequency resolution finer than the first analogue demodulating signal, first processing means for processing the radio frequency signal in combination with the first analogue demodulating signal to produce a first analogue intermediate signal, second processing means for processing the first analogue intermediate signal in combination with the second analogue demodulating signal to produce a second analogue intermediate signal, and adjusting means arranged in cooperation with the signal generating means to adjust the frequencies of the first and second demodulating signals in accordance with their respective frequency resolutions in order to tune the receiver to the radio frequency signal.
According to a fourth aspect of the present invention there is provided apparatus for transmitting a modulation signal in a multiple channel radio system, in which each channel has an associated channel frequency and the channel frequencies of adjacent channels are separated by a channel spacing, the apparatus comprising: signal generating means for producing a second analogue modulating signal with a frequency resolution wider than the channel spacing, and a first analogue modulating signal with a frequency resolution finer than the second analogue modulating signal, first processing means for processing the modulation signal in combination with the first analogue modulating signal to produce an analogue intermediate frequency signal, second processing means for processing the analogue intermediate signal in combination with the second analogue modulating signal to produce a radio frequency output signal, and adjusting means arranged in cooperation with the signal generating means to adjust the frequencies of the first and second modulating signals within their respective frequency resolutions so as to change the channel frequency of the output signal.
In one embodiment the signal generating means comprises a first synthesiser for producing the first analogue modulating or demodulating signal, and a second synthesiser for producing the second analogue modulating or demodulating signal.
Ideally, the first and/or second processing means comprises a mixing unit. Signals input to the processing means may be mixed by the mixing unit with a modulating or demodulating signal.
According to a fifth aspect of the present invention there is provided a method for receiving a radio frequency signal of a multiple channel radio system, in which each channel has an associated channel frequency and the channel frequencies of adjacent channels are separated by a channel spacing, the method comprising the steps of: processing the radio frequency signal in combination with a first analogue demodulating signal to produce a first analogue intermediate signal, and processing the first analogue intermediate signal in combination with a second analogue demodulating signal to produce a second analogue intermediate signal, wherein the frequency resolution of the second analogue demodulating signal is wider than the channel spacing, and the frequency resolution of the first analogue demodulating signal is finer than the frequency resolution of the second analogue demodulating signal, and the frequencies of the first and second analogue demodulating signals are adjusted in accordance with their respective frequency resolutions in order to tune the receiver to the radio frequency signal.
According to a sixth aspect of the present invention there is provided a method for transmitting a modulation signal in a multiple channel radio system, in which each channel has an associated channel frequency and the channel frequencies of adjacent channels are separated by a channel spacing, the method comprising the steps of: processing the modulation signal in combination with a first analogue modulating signal to produce an analogue intermediate signal, and processing the analogue intermediate signal in combination with a second analogue modulating signal to produce a analogue radio frequency output signal, wherein the frequency resolution of the first analogue modulating signal is wider than the channel spacing, and the frequency resolution of the second analogue modulating signal is finer than the frequency resolution of the first analogue modulating signal, and the frequencies of the first and second analogue modulating signals are adjusted in accordance with their respective frequency resolutions so as to change the channel frequency of the output signal.
The apparatus or the method in accordance with the invention may be suitably incorporated in a mobile, transportable or handheld radio telephone.